(1)在主动语态中,make , let , hear , see , watch , notice 等词,其后跟省略to的动词不定式(动词原形),在变为被动语态时,必须还原动词不定式符号to.如:
We heard him sing this song in the room.
He was heard to sing this song in the room.
The boss made him work 14 hours a day.
He was made to work 14 hours a day.